The present invention relates to a method for the preparation of hydrocarbons from synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, typically labeled the Fischer-Tropsch process. Particularly, this invention relates to a method for reducing the maximum water concentration in multi-phase column reactors operating at Fischer-Tropsch conditions.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into useful hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes, olefins, and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
There are continuing efforts to design reactors that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. Catalysts for use in such synthesis usually contain a catalytically active metal of Groups 8, 9, or 10 (in the New notation of the periodic table of the elements, which is followed throughout). In particular, iron, cobalt, nickel, and ruthenium have been abundantly used as the catalytically active metals. Cobalt and ruthenium have been found to be most suitable for catalyzing a process in which synthesis gas is converted primarily to hydrocarbons having five or more carbon atoms (i.e., where the C5+ selectivity of the catalyst is high).
Originally, the Fischer-Tropsch synthesis was operated in packed bed reactors. These reactors have several drawbacks, such as difficulty of temperature control, that can be overcome by using gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated reactors, sometimes called xe2x80x9cslurry reactors,xe2x80x9d xe2x80x9cslurry bubble columns,xe2x80x9d or xe2x80x9cmulti-phase reactorsxe2x80x9d operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and/or liquid products. If gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspended solid using any suitable technique, such as settling, filtration, magnetic separation, hydrocycloning, or the like, and then separating the liquids.
A known problem in multi-phase reactors, however, is that water is continuously formed during Fisher-Tropsch synthesis in the reactors. The presence of water limits conversion and prematurely deactivates catalyst systems such as iron and cobalt-based Fisher-Tropsch catalysts [e.g., Schanke et al., Catalysis Letter 34 (1995) 269; Hilmen et al., Applied Catalysis, 186 (1999) 169; van Berge et al., Catalysis Today, 58 (2000) 321). Thus, a high water partial pressure correlates to a high deactivation rate. In addition, it is believed that above a certain partial pressure of water, the catalyst deactivates faster. For example, some have observed that partial pressures of water above about 6 bar deactivate certain Fischer-Tropsch catalysts, while partial pressures of water below that level do not [Schanke et al., Energy and Fuels, 10 (1997) 867]. It is further believed that the relationship between deactivation rate and water concentration may have one or more thresholds, between which the relationship may or may not be linear. Furthermore, the relationship between deactivation rate and water concentration may depend on other physical parameters of the system. Regardless of the precise nature of the relationship, it is believed that reducing reactor water concentration would reduce the rate of catalyst deactivation.
For any given cobalt-based catalyst, along with the H2/CO ratio and the reaction temperature, the total pressure has a direct influence on the wax selectivity, in that a higher pressure will result in a higher wax selectivity. However, a higher total pressure (at any given degree of per-pass conversion) also correlates to a higher water partial pressure and therefore a higher deactivation rate. Therefore, if reactors are operated at conditions that are conducive to higher alpha values, per-pass conversion will necessarily have to be low to avoid premature catalyst deactivation due to water. A low per-pass conversion is undesirable, however, because it results in higher capital investment and operating costs. Additionally, for iron-based catalysts, the water not only has a negative effect on the catalyst deactivation rate, but it also inhibits the rate of reaction (see for example, Kirillov, V. A. et al., in Natural Gas Conversion V, Studies in Surface Science and Catalysis, vol. 119, A. Parmaliana et al., ed., Elsevier Science, New York, pp. 149-154, 1998).
The water partial pressure is therefore a constraint that prevents the realization of the kinetic and/or wax selectivity potential of iron and cobalt-based Fisher-Tropsch catalysts. Therefore, in order to improve the efficiency of multi-phase reactors using iron and cobalt-based catalyst systems, there exists a need for a method to reduce the maximum water concentration reached in the system during Fisher-Tropsch synthesis.
The present invention relates to a method for reducing the maximum water concentration in multi-phase reactors operating at Fischer-Tropsch conditions. More particularly, the present invention is based on the recognition of a high water concentration region and alteration of the flow patterns within the reactor in order to reduce the maximum water concentration in the reactor. This method increases the catalyst lifetime, thereby reducing the operating cost of the Fischer-Tropsch process.
In a preferred embodiment of the present invention, a method of reducing the maximum concentration of water in a multi-phase reactor containing an expanded slurry bed and a water-rich slurry region for Fisher-Tropsch synthesis includes changing the flow patterns of the fluids within the reactor and/or diluting the water concentration in the high water concentration region. More precisely, flow patterns are modified so as to cause a mixing of fluids from a water-rich region in the reactor with fluids in the rest of the reactor. The flow patterns may be changed by introducing a mixing enhancing fluid into the predetermined region, installing baffles into the predetermined region, or the combination of the two, or by other mechanical mixing methods known in the art. The introduction of the mixing enhancing fluid can also lead to a dilution of the water concentration in the predetermined region. In some reactors, the water-rich region is located between xc2xd H and H and between xc2xd R and R, where H is the height of the expanded slurry bed and R is the radius of the reactor. The expanded slurry bed is herein defined as the region within a reactor where an intimate liquid-solid-gas phase contact exists. The flow patterns are preferably changed or disrupted so that the difference between the highest water concentration in the reactor and the lowest water concentration in the reactor is minimized.
The present invention allows higher per-pass conversions of syngas and/or use of higher total pressures at any degree of conversion, while protecting the Fischer-Tropsch catalyst from an excessive oxidation rate.